Traditionally, sound attenuators or mufflers are used to reduce unwanted sounds in ducts with flowing air. Such a sound attenuator or muffler is usually designed as an add-on component, inserted somewhere along the duct. Among sound attenuators, there are resistive and reactive types.
A purely reactive type reduces the sound only by means of a difference regarding the geometrical dimension in relation to the duct, and can be regarded as a reflector and not an energy dissipative silencer.
A resistive silencer often contains resistive materials like pressed glass or mineral fibre wools, plastic foams, or polyester fibres as energy absorbing material. Thereby, placing the absorbing material, in the form of baffles or sidewall linings, in the flowing stream is the most effective way to reduce sound, since the sound is present in the stream. However, this will inevitably cause unwanted effects like a pressure drop, noise generated by turbulence, and reduced volume flow.
In order to avoid or reduce a pressure loss in the duet of a resistive silencer, wall-linings can be placed outside the original duct, whereby the duct has to be partly perforated in order to let the sound dissipate into the porous material, and an airtight wall has to be provided behind the absorbing material in order to avoid pressure loss and leakage. However, a disadvantage with such a solution is that it requires space outside the stream flow duct, and such space can be limited or non-existing in many applications.
Other solutions include a baffle in a sound absorbing material inserted in the stream flow duct, or a vane inserted in the stream flow duet to reduce turbulence. Such vanes are often provided in ducts bends.
WO02/089110 discloses a silencer design using microperforated linings on walls and baffles in one or several layers, using no fibrous absorbing materials. The air flow passes parallel to the surface of the using the non-moving air inside the lining or the baffle as a non-locally reacting absorber. A disadvantage with this solution is that the volume behind the linings or the baffles does not participate as a flow duct, but rather reduces the flow cross-section area.
GB1536164 describes an acoustic attenuator using angled vanes in a straight duct in order to guide the airflow and the sound field. The vanes form a plurality of flow channels distributed both in the transverse and longitudinal direction of the duct. The sound field is guided so that the sound waves impact against many vanes, which are covered with a porous material, so that the waves loose energy on every impact. Acoustically the vanes are mainly reflective. However, due to the angle of the vanes, and the large thicknesses of the vanes and the absorption layers, the vanes will cause a reduction of the flow cross-section. Further, the bending of the flow to a zigzag shape will cause a pressure drop.